1. Field of the Invention
The present invention relates to a friction stir welding method and its welding device in which jointing portions to be jointed of metal work piece are stirred to be flown in a plastic state due to friction heat generated by stirring and then the metal structure of the jointing portions is cooled down to be unified.
2. Description of the Related Art
Friction stir welding is a solid-state joining process without using fusion or filler materials, thereby the original metal characteristics being maintained to be unchanged as far as possible.
A conventional friction stir welding method of this kind is disclosed in Japanese Patent N0. 4215179. This prior friction stir welding method has a tool with a small pin or nib, a drive electric motor that rotates the tool, an air cylinder that presses the tool on jointing portions of a work piece, and slide mechanisms that moves the tool on a horizontal surface (X-axial direction and Y-axial direction) and in a vertical direction (Z-axial direction). The tool has a shoulder portion, which is larger in radiator than the pin. The shoulder and the pin are pressed on the jointing portions, being rotated, so as to be inserted into them, and the tool is moved along the jointing portions in a state where a center axis of the tool is slanted along the moving direction so that the pin side is positioned forward.
However, in the above known conventional friction stir welding method, there is a problem in that the joining temperature in the jointing portions might reach the melting points of the metals and/or the durability of the pin deteriorates due to the friction stir welding that is carried out at the jointing portions in a case where the straight sections are contacted with each other through a square corner or through a small round corner.
The reason is as follows.
Normally the tool and pin are rotated at a constant speed and they are moved along a weld line (the portions to be jointed) of work piece at a constant speed, where the rotating pin penetrates the portions to be jointed to the depth slightly less than the weld depth required and the rotating tool shoulder is in contact with the portions to be joined. This mechanical mixing process generates heat, which causes the stirred material to soften without reaching its melting point.
When the above-mentioned friction stir welding method is applied to a work piece which has straight sections connected with each other through a tight corner such as a square corner or a small round corner, the tool with pin is needed to change its moving direction at the corner. In this case, the tool with pin is rotated on the corner at a high total speed consisting of a rotating speed of the tool with pin and a direction changing speed thereof. Consequently, the heat generated on the corner rises to have a temperature higher than that generated at the straight sections of the work piece. This higher heat might cause the metal characteristics of the jointed portions to be changed due to the temperature higher than the melting point of the work piece, and/or it might deteriorate the durability of the pin.
It is, therefore, a first object of the present invention to provide a friction stir welding method which overcomes the foregoing drawbacks and can weld portions to be joined of a work piece as fast as possible, avoiding the temperature of the portions to be jointed of the work piece from reaching the melting point of the work piece at a tight corner such as a square corner and a small round corner during friction stir welding.
It is a second object of the present invention to provide a friction stir welding device which overcomes the foregoing drawbacks and can weld portions to be joined of a work piece as fast as possible, avoiding the temperature of the portions to be jointed of the work piece from reaching the melting point of the work piece at a tight corner such as a square corner and a small round corner during friction stir welding.